Add window.storage to the background page, which loads all data from the
'items' store in indexeddb, caching them in memory for synchronous
access, then override textsecure storage to use that in memory store.
Update unreadCounts per-conversation on incoming messages. Render unread
conversations with font-weigh: bold in the inbox view.
To ensure that the inbox and conversation views remain in sync, the
background page now ensures that the same models objects are used for
both views.
